One thing to note is that TWikiAdminGroup does not necessarily determine whether a user is an admin or not.
By default, that's the case since TWiki::Users::TWikiUserMapping::isAdmin() behaves that way.
A while back, I introduced TWiki::Users::isAdmin(), which calls the isAdmin() of the currently effective user mapping.

So now it's easier than before to introduce the isadmin context. It's only a matter of calling TWiki::Users::isAdmin().